Atmospheric pressure ionization mass spectrometers analyze trace impurities (on the level between parts per billion (ppb) and parts per trillion (ppt)) in gases. These instruments are used routinely to analyze gas samples for water and in manufacturing of semiconductor devices where analysis of trace impurities and supply of highly purified gases in the production process is crucial to device performance.
Atmospheric Pressure Ionization Mass Spectrometry (APIMS) differs from conventional mass spectrometry in that the ions are generated near atmospheric pressure. In electron impact ionization the source pressure is maintained below about 2 torr. At these pressures the ions observed are those generated by only one collision because the collision rate is so low that chemical reactions between ions are slow. In contrast the collision rate near atmospheric pressure is very high so primary ions rarely survive. The ions observed are the result of chemical reactions of ions and neutrals. When the ions enter the mass spectrometer they may or may not be near equilibrium so the concentrations of ions can be influenced by both the kinetic and equilibrium properties of the system.
A conventional atmospheric pressure ionization mass spectrometer is described in U.S. Pat. No. 4,023,398. In this apparatus, sample gas at almost 1 atmosphere is ionized in an ion source by discharge or radiation. Formed sample gas ions cause an ion-molecule reaction and trace components contained in the sample gas are ionized highly efficiently. Formed trace component ions are transmitted by the electric field through a gas curtain chamber and an aperture to an analysis region wherein they are separated by mass and detected. To prevent sample gas from entering the analysis region, the curtain gas is controlled so as to flow into the ion source from the gas curtain chamber.
U.S. Pat. No. 5,304,797 describes a spectrometer having an ion source chamber and a drift chamber for drifting and separating the main component ions of a gas sample from impurity ions. Ionization means for this spectrometer may comprise a corona discharge, a radiation source, a laser or any other known and suitable ionization means.
Positive ion APIMS using corona discharge ion sources are widely used for determining trace contamination in industrial gases. It is the standard for determining moisture and is the method to which all other techniques are compared. A simplified drawing of a corona discharge source is shown in FIG. 1. The needle is maintained at about 5000 volts and is located about −2 mm from an insulated metal orifice whose potential is controlled near ground potential. In positive ion mode, electrons stream from the orifice to the needle producing a neutral ion plasma. Under relatively dry conditions, low analyte concentrations and when other species of higher proton affinity are at low concentration, positive ion APIMs can be used to determine many species of interest including, but not limited to CH4, CO2, and O2.
However, when other species of higher proton affinity are present in relatively high concentrations, for example in the determination of H2O content in NH3 gas, positive ion APIMS is difficult to employ. Specifically, since the proton affinity of NH3 is so much higher than H2O, ions produced from H2O normally cannot be observed. Further, peaks for the adducts of H2O are weak because the NH3 concentration is so high that NH4+ and its complexes dominate the spectrum.
In negative ion mode the needle of the corona discharge is positive so electrons stream from the needle to the orifice. However, with most sources of electrons, a large fraction of the electrons enter the mass spectrometer. The electrons pass through the quadrupole filter without being deflected producing a large background signal at each mass. This occurs, although to a lesser degree, even when substances of finite electron affinity are present. The large background signal created by these electrons entering the mass spectrometer increase the lower limit of detection and decreases sensitivity.